Testing: cts: add target-by-attribute to the random fencing configurations

Previously, CTS would choose randomly from "broadcast" (no topology),
"levels-and" (FencingPass and Fencing in same topology level),
and "levels-or" (FencingFail and Fencing in successive topology levels)
when configuring node fencing, always targeting topology by node name.

Now, if it chooses "levels-and", it also randomly chooses between
targeting by node name and targeting by node attribute.